Comparison of Oxide Film Growth and Relationship to Subsurface Chemistry and Hydrogen Ingress in New Generation 7xxx Series Al-Aerospace AlloysThursday (07.11.2019) 09:55 - 10:15 Part of:
One of the major trends in the development of high strength, new generation, 7xxx Al alloys is an increase in Zn levels, and the Zn to Mg ratio, which has been found to decrease the quench sensitivity of thick plate material and therefore allow for more homogenous materials properties in larger section components. However, evidence is now emerging that new generation thick plate alloys may be more susceptible to environmentally assisted cracking (EAC) in humid air environments, although the precise role of Zn within this phenomena is currently unclear. Here, we have used a range of complementary techniques to characterize and compare the morphology and chemistry of the surface hydrated oxide layers that form in these alloys and related this to the hydrogen uptake into the metal. It has been observed that after exposure to humid air at elevated temperatures (50 ⁰C) both older lower-Zn content alloys and newer generation alloys form a stable passive layer of thickness ~ 10-15 nm. High resolution chemical mapping of PFIB-prepared cross sectional samples using STEM-EDX has revealed the presence of Mg within the passive layer in both alloys and surface segregation of Cu to the interface between the layer and substrate. A major difference observed between the new and more established alloys is stronger segregation of Zn to the interfacial region, which is correlated to the Cu segregation. Qualitative measurement of local sub-surface hydrogen distribution has also been performed using glow discharge optical emission spectroscopy (GDOES), which has consistently shown higher levels of absorbed hydrogen in the sub-surface region in the higher Zn content alloys. This work has also supported the TEM analysis and confirmed the presence of both Zn and Cu enrichment in the interfacial region that was not observed in lower Zn alloys after 4 months exposure. It is postulated that the higher Zn levels, in particular in the alloy matrix, can lead to doping of the usually protective inner layer of the hydrated films, increasing the rate of hydrogen ingress in these alloys.